Exposure to lead in early life produces long-lasting developmental delay. It is not known whether pharmacologically lowering blood lead prevents or ameliorates subsequent delays, and, if it does, whether it can be done with an acceptable degree of safety. The Toxicity of Lead in Children Trial is a multi-center, placebo-controlled, clinical trial comparing the use of the oral chelating drug succimer with placebo in the prevention of developmental delay, slowed growth, and behavior disorders in toddlers. All children will receive home clean up and vitamin and mineral supplementation. Half will receive active drug and half placebo. Enrollment of children began in August 1994; all 800 children should be enrolled in about two years. The children will be between 12 and 32 months of age, and have blood lead levels between 20 and 44 micrograms per deciliter. All children will be followed for 3 years, and their development and behavior measured. The trial is designed on the assumptions that peak blood leads, achieved in the US by children at about age 2 years, cause developmental delays in 4 year olds, and that succimer will lower blood leads in the range of eligibility of the trial. Quantitatively, a 10 microgram per deciliter change in blood lead is expected to result in a 3 point difference in measured IQ. The trial aims for a mean blood lead difference between children given active drug and placebo of about 9 micrograms per deciliter.